If you used "wa" in this sentence, it makes it sounds like knowledge is a person (state of being) or well-known fact. Similar example would be "地球は丸い" vs "地球が丸い." It is a known fact that the earth is round, but if you were Galileo back in 1500's when the earth was "not round," you could say the latter. Since the wishful knowledge/lack of knowledge is not a known fact to most people (knowledge often gives you power), "ga" is appropriate in this context.